I like him. I unfortunately see him ending up as another Diesel/Lex Luger, but I'm at least willing to give him a chance before I dump on him completely. He's over, his merch is doing real well, he's got the crowd's support, he's young, hungry and eager to improve, he isn't bad in the ring, etc. His mediocre verbal skills are his only big flaw, but that can easily be masked by not booking him to go out there and cut lengthy promos, at least not until he improves in that area.
They're kinda rushing him to the top a bit but sometimes you gotta take a chance. Not everyone has to languish in the middle of the card for a set amount of time before being pushed to the top. There's no set formula for someone becoming a main event star. Batista was a midcarder doing nothing in Evolution and it was only four months after they began teasing tension between he and HHH that he became a main event star for life. Putting the title on Goldberg less than a full year after he debuted didn't exactly do him any harm. Nothing says that magic can't happen with Reigns by having him go over Lesnar.
And to be fair, who else would be a better choice to defeat Brock? Bryan likely won't be back in time, Ziggler is never gonna be that guy, Rollins is a heel, Ambrose has still yet to win a single noteworthy match and isn't anymore popular Reigns at the moment, Orton is already a huge star with numerous accolades to his name and doesn't need a big one of this kind, lol @ Ryback, Punk just signed with UFC and isn't coming back anytime soon, etc.